From SHOtimes - this should clear any questions up.
From Gary Morrell:
Economics, personal style, and availability have a lot to do with this behaviour. The SHO is only part hot rod, the rest is conventional 4-door sedan, and that's how most of its buyer's use it. Folks like me who's SHO is modified to the point of barely being roadable are a tiny, tiny minority. I see plenty of SHO's with 2 baby seats strapped in the back, which would get in the way of the attachment points for my driver and passenger 5-point harnesses. ;-)
At high altitude, fuel runs from 85 to 91 octane, not the 87 to 93 that flatlander's are used to. The 20% thinner air reduces the engine's effective compression ratio so high octane fuels simply aren't useful.
If you look in the SHO owner's manual, on the dash, and on the gas door sticker, it says "Premium fuel recommended", not required. All Ford engines, regardless of the intended performance level, are calibrated on regular fuel (87 octane), not premium. The reason for this is to obtain a calibration that will give optimum driveability under a wide variety of environmental conditions and available fuels.
Because the Yamaha engine is not particularly octane-limited, meaning that the compression ratio is fairly reasonable and the engine can tolerate large amounts of spark advance, (which helps to make more power) the difference between 93 and 87 octane would probably mean an approximate 3% decrease in low RPM torque under adverse conditions: like a fully loaded car and 95degF ambient temperature. Remember, higher octane fuels do not produce any more power when burned; octane is a measure of a fuel's ability to resist premature ignition when subjected to heat and pressure. The higher the octane number, the less likely a fuel is to pre-ignite. High octane fuels allow engines to produce more power because they allow more spark advance before pre-ignition occurs. It is of no benefit to put 100 octane fuel into an engine that doesn't have the spark advance dynamic range to take advantage of it.
Under warm cruise (closed loop) strategy, EEC will push the spark advance out to the point where trace knock is detected, and then back the timing off slightly, to keep away from pre-ignition. This strategy makes sense to get the most efficient burn and the most power. Under these conditions, you may hear some knock if you mash the gas, because it takes EEC a few CPU machine cycles and crankshaft rotations to drop out of closed-loop strategy and retard the timing.
Under wide-open-throttle (WOT) conditions, EEC is not looking at the knock sensor because the engine is simply making too much noise for the knock detection strategy to filter out the knock signal from the noise. Timing and fuel for WOT is derived from lookup tables that are vectored primarily by crankshaft RPM and engine coolant temperature, and to a much lessor degree by the mass air signal.
Under most conditions, EEC should be able to get the most out of any reasonable fuel that you pump into the tank, however, an especially bad load might be beyond the range that the spark lookup tables can compensate for, so pulling the octane shorting bar invokes a new set of tables, with less aggressive spark advance curves for WOT operation.
<small>[ November 12, 2002, 04:16 PM: Message edited by: twrsho ]</small>
